Novel iron(ii) complexes with hexadentate nitrogen ligands obtained via intramolecular redox reactions

Abstract

Two novel complexes: [Fe(L^2′)][BPh₄]₂, 1, and [Fe(L^3′)][BPh₄]₂, 2, with the hexadentate nitrogen ligands, L^n′ = 1,9-bis(2′-pyridyl)-5-[(R-2″-pyridyl)methyl]-2,5,8-triazanon-1-ene, where R = ethoxy for L^2′ and methoxy for L^3′, were obtained from the iron(III) complex of the pentadentate ligand, L¹ = 1,9-bis(2′-pyridyl)-2,5,8-triazanonane. Complexes 1 and 2 were also obtained by making the hexadentate ligands: 1,9-bis(2′-pyridyl)-5-[(ethoxy-2′-pyridyl)methyl]-2,5,8-triazanonane (L²) and 1,9-bis(2′-pyridyl)-5-[(methoxy-2″-pyridyl)methyl]-2,5,8-triazanonane (L³) react with Fe(III), respectively. The structures of complexes 1 and 2 were characterized by COSY, HMBC, HMQC and NOESY NMR studies, and both structures were also confirmed by X-ray analysis. In both cases, the geometry around iron is a distorted octahedron. Since 1 and 2 are diamagnetic at 298 K they are low-spin iron(II) species. Both preparative methods are examples of oxidative dehydrogenation of a Fe(III) polyamine complex, in which the thermodynamically and kinetically stable final product is a low spin Fe(II) imine complex. In the case of the first method an increase in the size and denticity of the starting ligand is observed.